Since its discovery in 1899, it has been well known that via the Baeyer-Villiger reaction, ketones may be oxidized to esters by means of a percarboxylic acid: ##STR1## The mechanism of the reaction is such that a complex peroxidized intermediary is formed and subsequently rearranges itself according to an ionic process to yield an ester.
A particularly important application of the Baeyer-Villiger reaction is in the preparation of lactones from cyclic ketones: ##STR2## In the above reaction, when n=3, the cyclic ketone is cyclohexanone and its oxidation by percarboxylic acid yields .epsilon.-caprolactone.
It is well known that .epsilon.-caprolactone is unstable in an acidic medium. Therefore, in order to obtain this particular lactone in high yields directly from cyclohexanone via the Baeyer-Villiger reaction, it is essential that the solution of percarboxylic acid used demonstrates the weakest possible acidity. Due to this requisite factor of low acidity, the choice of method for the preparation of percarboxylic acid becomes important.
One proposed method involves the use of peroxyacetic acid which is prepared by the oxidation of acetaldehyde by oxygen. The major inconvenience of this process is the necessity of valorizing the acetic acid co-product formed during the oxidation of cyclohexanone.
Another proposed method reacts a carboxylic acid with hydrogen peroxide to prepare percarboxylic acid. Advantageously, this type of process permits the recycling of carboxylic acid after the oxidation of cyclohexanone. However, percarboxylic acid solutions obtained by the reaction of hydrogen peroxide with a carboxylic acid according to known methods all demonstrate strong acidity. This is due to the fact that the carboxylic acid used is a strong acid, such as formic acid, or that the formation of percarboxylic acid is catalyzed by a strong acid catalyst, such as sulfuric acid.
Thus, the oxidation of cyclohexanone by solutions of percarboxylic acid obtained according to these known methods necessitates neutralization of the strong acid catalyst in order to avoid adverse effects in the production of .epsilon.-caprolactone and the formation of numerous by-products. The inclusion of a neutralization step, however, leads to the precipitation of mineral salts, which are difficult to separate from the medium and which are frequently, themselves, catalysts for the degradation of .epsilon.-caprolactone.
To avoid these inconveniences, Belgium Pat. No. 540,412 and British Pat. No. 776,758 have proposed the use of an ion-exchanging resin, having strong acid groups, as a catalyst for the formation of percarboxylic acid. This catalyst is very effective and is easy to separate, but it presents a danger in that heavy metallic ions, such as Fe.sup.+3 ions, which are known to be catalysts for the violent decomposition of percarboxylic acids and are always present in a small concentration in the organic products, become fixed to and accumulated on the resins.
Other proposed methods use pure percarboxylic acid obtained by distillation. However, these processes also present evident dangers.